logger: Add custom logging capabilities

Adds the ability for users to log to custom log levels
by providing custom log level names in logger.conf. Also
adds a logger show levels CLI command.

ASTERISK-29529

Change-Id: If082703cf81a436ae5a565c75225fa8c0554b702

configs/samples/logger.conf.sample

@@ -85,6 +85,11 @@
 ; The default is 1000
 ;logger_queue_limit = 250
 ;
+; Any custom logging levels you may want to use, which can then
+; be sent to logging channels. The maximum number of custom
+; levels is 16, but not all of these may be available if modules
+; in Asterisk define their own.
+;custom_levels = foobar,important,compliance
 ;
 [logfiles]
 ;
@@ -130,6 +135,7 @@
 ;    dtmf
 ;    fax
 ;    security
+;    <customlevel>
 ;
 ; Verbose takes an optional argument, in the form of an integer level. The
 ; verbose level can be set per logfile. Verbose messages with higher levels
@@ -176,3 +182,5 @@ messages => notice,warning,error
 ;
 ;syslog.local0 => notice,warning,error
 ;
+; A log level defined in 'custom_levels' above
+;important.log = important