Refactor: Remove unnecessary calls to pcmk_strerror.

This function is deprecated, and many uses can be replaced with calls to
pcmk_rc_str instead. These fall into two categories:

• pcmk_strerror(errno) - As long as errno is positive, this fits into the new standard return code space so we can just call pcmk_rc_str instead.

• if (rc != pcmk_rc_ok) { pcmk_strerror(rc); } - These are obvious. The new standard return codes can just be passed to pcmk_rc_str.

There are plenty of pcmk_strerror calls still around, but there's plenty
of places still using the old style return codes.