New command for showing multiple log files

Also new function syslog-get-basename-and-number used by
syslog-previous-file and syslog-open-files.
Also new customization option syslog-log-file-directory.

Altered items:
syslog-mode-map, syslog-get-basename-and-number,
syslog-open-files, syslog-previous-file, syslog-log-file-directory
Add keybindings to quit and to open files

Also make sure syslog-mode sets the buffer as read-only.

Altered items:
syslog-mode-map, syslog-mode
Alter syslog-open-files: reuse existing buffer with same name

Altered items:
syslog-open-files