~bitfehler/m2dir

Specify encoding for folder names if required
Use IANA names for standard flags

They were already the reference anyways, and just using the IANA name is
a much more neutral approach. The `$Deleted` IMAP flag was pulled in
because it is likely to occur often in the wild.
Use comma instead of colon as UID separator

The comma has two slight advantages over the colon:

* Bash completion for some reason wants to escape colons, making their
  usage a bit awkward at times
* Colons are not usable on NTFS due to ADS [1]

Combined, this makes a compelling case to use the comma instead, which
has none of these issues.

[1] https://en.wikipedia.org/wiki/NTFS#Alternate_data_stream_(ADS)
Fix heading id collision
Various updates to specification

These updates are mostly a result of this discussion:

https://lists.sr.ht/~bitfehler/m2dir/%3C17c32401239f7b0b.f35d3f6599202235.cdc008ecf268f5a1@mba%3E

In addition, flags were restricted to not contain control characters.
Some minor spelling fixes for consistency.