bash pollutes namespace it shouldn't touch when emulating sh

[Neven Sajko]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64'
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-unknown-linux-gnu'
-DCONF_VENDOR='unknown' -DLOCALEDIR='/usr/share/locale'
-DPACKAGE='bash' -DSHELL -DHAVE_CONFIG_H   -I.  -I. -I./include
-I./lib  -D_FORTIFY_SOURCE=2 -march=x86-64 -mtune=generic -O2 -pipe
-fstack-protector-strong
-DDEFAULT_PATH_VALUE='/usr/local/sbin:/usr/local/bin:/usr/bin'
-DSTANDARD_UTILS_PATH='/usr/bin' -DSYS_BASHRC='/etc/bash.bashrc'
-DSYS_BASH_LOGOUT='/etc/bash.bash_logout'
-DNON_INTERACTIVE_LOGIN_SHELLS -Wno-parentheses -Wno-format-security
uname output: Linux hp 4.12.8-2-ARCH #1 SMP PREEMPT Fri Aug 18
14:08:02 UTC 2017 x86_64 GNU/Linux
Machine Type: x86_64-unknown-linux-gnu

Bash Version: 4.4
Patch Level: 12
Release Status: release

Description:

Some bash features (history expansions ...?) prevent executing
commands with the same name.

Repeat-By:

$ echo x"$histchars"x
xx
$ POSIXLY_CORRECT=1 sh
sh-4.4$ ^
sh: !!: event not found
sh-4.4$ histchars=,\\   # This should not have to be done...
sh-4.4$ ^
I, too, am in PATH!
sh-4.4$ # This was how it should have been in the first place!
sh-4.4$ unset histchars
sh-4.4$ ^
sh: :s^: no previous substitution

Fix:

Just remove those features while emulating sh. Or maybe, like it is
done for bashbug, check that there isn't a matching executable in
PATH.