Disable ability to load PKGBUILD from #stdin
scripts/library/parseopts avoid un "unbound variable"
makepkg: fix "unbound variable" issues
makepkg#check_pgpsigs Fix typo error  (warning -> warnings)
makepkg: prevent the script from being invoked directly

The #makepkg script can only be invoked by #s-makepkg.
makepkg: fix "unbound variable" issues
makepkg: use #_pkgbuild_load to modify some internal variables