Stop PKGMAN_Exec from parsing its arguments

[fingolfin]

Claude writing here:

PKGMAN_Exec joins its arguments with spaces and runs the result with sh -c, so an argument with a space is split and one with $(...) or ; is executed. Archive names are the last component of a PackageInfo.g's ArchiveURL, so a crafted URL runs code at tar xf.

gap> PKGMAN_Exec(".", "printf", "[%s]", "a  b", "$(echo injected)").output;
"[a][b][injected]"          # before
"[a  b][$(echo injected)]"  # after

The shell now gets command and arguments as positional parameters (sh -c 'exec "$0" "$@" 2>&1' cmd arg...): it still merges stderr, but never parses them. #104 is unaffected.

Two callers passed "" to mean "no option" (non-GNU tar, hg status); they now pass nothing.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.14%. Comparing base (9e40dc4) to head (15d33cf).

Files with missing lines	Patch %	Lines
gap/archive.gi	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
+ Coverage   85.12%   85.14%   +0.01%     
==========================================
  Files          22       22              
  Lines        1022     1023       +1     
==========================================
+ Hits          870      871       +1     
  Misses        152      152              

Files with missing lines	Coverage Δ
gap/PackageManager.gi	94.80% <100.00%> (ø)
gap/archive.gi	92.42% <75.00%> (+0.11%)	⬆️

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[mtorpey]

I'm having a bit of trouble with this. What is the problem that this PR is attempting to solve, and what is the intention of Concatenation(["-c", "exec \"$0\" \"$@\" 2>&1", fullcmd], on line 302?