cephconfig: align our parser with Ceph's parser

cephconfig: align our parser with Ceph's parser

This commit rewrites the entire parser for ceph.conf, aligning its
behaviour as closely as possible with Ceph's parser grammar [0].

The most notable improvements are as follows:

  1. The characters '#' and ';' now both mark comments, instead of
     just the '#' character.

  2. Any character, including comment literals ('#' and ';'), may now
     be escaped.

  3. Quoted values (single and double) are now supported.

  4. Line continuations are now supported (lines ending with '\').

  5. Repeated whitespace characters in keys are now treated as a
     single space character.

  6. Dashes '-' are not treated the same as spaces and underscores
     anymore, as Ceph's grammar doesn't treat them that way.
     * Paired with 5., this means that repeated whitespace is now
       equivalent to a single underscore.

  7. Escaped comment literals are now un-escaped.

  8. Although not too crucial, the parser now also supports empty
     sections and will just initialize them with an empty hash.

Furthermore, the original grammar's more quirky behaviours are also
respected where sanely possible.

[0]: https://git.proxmox.com/?p=ceph.git;a=blob;f=ceph/src/common/ConfUtils.cc;h=2f78fd02bf9e27467275752e6f3bca0c5e3946ce;hb=e9fe820e7fffd1b7cde143a9f77653b73fcec748#l144

Signed-off-by: Max Carrara <m.carrara@proxmox.com>
Tested-by: Friedrich Weber <f.weber@proxmox.com>