sh: Allow more scripts without #!
Austin Group bugs #1226 and #1250 changed the requirements for shell scripts
without #! (POSIX does not specify #!; this is about the shell execution
when execve(2) returns an [ENOEXEC] error).
POSIX says we shall allow execution if the initial part intended to be
parsed by the shell consists of characters and does not contain the NUL
character. This allows concatenating a shell script (ending with exec or
exit) and a binary payload.
In order to reject common binary files such as PNG images, check that there
is a lowercase letter or expansion before the last newline before the NUL
character, in addition to the check for the newline character suggested by
(cherry picked from commit e0f5c1387df23c8c4811f5b24a7ef6ecac51a71a)