Garbage in `eacces' caused the wrong errno to be set for non-EACCES errors.
Garbage in `etxtbsy' caused a semi-random retry strategy for ETXTBSY errors.
Found by: NIST-PCTS. gcc -Wall reported the problem, but -Wall is not
enabled for libc.
pointer returned by realloc(). All callers free the pointer if the
execve fails. Nuke the caching. This essentially restores buildargv()
to the 1.1.5 version. Also fix a memory leak if realloc() fails. Also
nuke similar but non-broken caching in execvp(). malloc() should be
efficient enough.