Auto merge of #278 - lemonrock:syslog, r=alexcrichton

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 `#define`s `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.