asterisk.c: Use the euid's home directory to read/write cli history

asterisk.c: Use the euid's home directory to read/write cli history

The CLI .asterisk_history file is read from/written to the directory
specified by the HOME environment variable. If the root user starts
asterisk with the -U/-G options, or with runuser/rungroup set in
asterisk.conf, the asterisk process is started as root but then it
calls setuid/setgid to set the new user/group. This does NOT reset
the HOME environment variable to the new user's home directory
though so it's still left as "/root". In this case, the new user
will almost certainly NOT have access to read from or write to the
history file.

* Added function process_histfile() which calls
  getpwuid(geteuid()) and uses pw->dir as the home directory
  instead of the HOME environment variable.
* ast_el_read_default_histfile() and ast_el_write_default_histfile()
  have been modified to use the new process_histfile()
  function.

Resolves: #337