Fix out of bounds access when running POSIX functions

[JohnoKing]

This commit salvages a (long in LOC) bugfix from the currently postponed local-builtin branch. The bug in question has previously been described in the following ksh2020 bug report: att#1423

The ksh2020 bugfix in 9490d10 does fix the out of bounds access, but I've opted to avoid backporting it because it uses malloc wastefully. On 01-01-2024 in 7721eeb I undertook the task of fixing this bug differently by moving POSIX function execution from b_dot_cmd() to the more appropriate sh_funscope(). Initially, this was primarily intended to aid in implementing dynamic scopes.

For this commit I have used my previous work on that branch, but have been careful to avoid changing scoping behavior in any way whatsoever (this patch primarily aims to fix the out of bounds behavior). Perhaps this approach is a bit overzealous, but I think this route is better because it provides an improved framework for tackling scoping problems, (see #123), which makes this manner of fix better longterm. It's also better for SSOT to have the function execution code concentrated in sh_funscope with if statements clearly demarcating those parts peculiar to POSIX functions and codepaths especial to KornShell functions. (It's not like this patch is brand new either; I should've submitted this fix standalone over a year ago. Better late than never, I suppose.)

Overview of changes:

• Move POSIX function execution out of b_dot_cmd() into sh_funscope. Due to the nature of dotted KornShell functions, those are still executed in b_dot_cmd.
• Delineate the codepaths for POSIX and KornShell functions in sh_funscope using if statements.
• Eliminate of the profitless DOTMAX definition; MAXDEPTH is adequate.
• With the above changes, simplify sh_funct() by doing away with the now needless b_dot_cmd call and out of bounds access.

As an aside, these changes slightly improve POSIX function performance (but only because POSIX functions lack scoping).
Arbitrary benchmarks:

$ cat bench/v-posix-fun.ksh
for ((i=0; i!=1000000; i++)) do
    fun() { :; }
    fun foo bar args
    unset -f fun
done
$ cat bench/v-posix-fun-less.ksh
for ((i=0; i!=1000000; i++)) do
    fun() { :; }
    fun
    unset -f fun
done

Results when run under shbench with 20 iterations and CCFLAGS=-O2 ('devbranch' is unpatched, 'basicfix' is u+m with the ksh2020 fix applied, 'funscope-overhaul' is u+m with this patch):

--------------------------------------------------------------------------------------------
name                  /tmp/ksh-devbranch    /tmp/ksh-basicfix     /tmp/ksh-funscope-overhaul
--------------------------------------------------------------------------------------------
v-posix-fun.ksh       1.125 [1.111-1.159]   1.159 [1.140-1.205]   0.966 [0.948-0.990]
v-posix-fun-less.ksh  1.115 [1.099-1.130]   1.141 [1.117-1.226]   0.946 [0.929-0.971]
--------------------------------------------------------------------------------------------