Low: libcrmcommon: Validate env option strings

• Check for NULL or empty option name in get/set functions; check for '=' in set.
• Check snprintf() return value for truncation or output error.
• Check setenv()/unsetenv() return value for failure.
• Check for NULL daemon name in pcmk__env_option_enabled() before passing to strstr().

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>