Add support for log masks

[belamenso]

Added functions setlogmask() and getlogmask() in order to implement functionality provided by syslog setlogmask (https://linux.die.net/man/3/setlogmask). Differences between this implementation and original:

• I used set[SyslogSeverity] instead of bitmasks,
• I opted for a new function, getlogmask(), instead of passing 0 to setlogmask(), which allows blocking all logging attempts and makes the interface more intuitive.