Adding syslog functions, constants and structs

This commit adds the functions:-
- syslog
- openlog
- closelog
- setlogmask

It adds LOG_* constants.

It adds the CODE struct used by the #define definitions prioritynames and facilitynames.

It does not add:-
- the function vsyslog; this is an extension to POSIX and uses va_list macros;
- the #defines prioritynames and facilitynames, as usage is not common
- rust functions mirroring the macros:-
  - LOG_PRI
  - LOG_MAKEPRI
  - LOG_MASK
  - LOG_UPTO
  - LOG_FAC
* CODE is included in case a third-party unsafe C function returns or takes it.