config: Make text_file_save and 'dialplan save' escape semicolons in values.

When a config file is read, an unescaped semicolon signals comments which are
stripped from the value before it's stored.  Escaped semicolons are then
unescaped and become part of the value.  Both of these behaviors are normal
and expected.  When the config is serialized either by 'dialplan save' or
AMI/UpdateConfig however, the now unescaped semicolons are written as-is.
If you actually reload the file just saved, the unescaped semicolons are
now treated as start of comments.

Since true comments are stripped on read, any semicolons in
ast_variable.value must have been escaped originally.  This patch
re-escapes semicolons in ast_variable.values before they're written to
file either by 'dialplan save' or config/ast_config_text_file_save which
is called by AMI/UpdateConfig. I also fixed a few pre-existing formatting
issues nearby in pbx_config.c

Tested-by: George Joseph
ASTERISK-20127 #close

Review: https://reviewboard.asterisk.org/r/4132/
........

Merged revisions 427275 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@427276 f38db490-d61c-443f-a65b-d21fe96a405b