app_voicemail.c: Completely resequence mailbox folders.

app_voicemail.c: Completely resequence mailbox folders.

Resequencing is a process that occurs when we open a voicemail folder
and discover that there are gaps between messages (e.g. `msg0000.txt`
is missing but `msg0001.txt` exists). Resequencing involves shifting
the existing messages down so we end up with a sequential list of
messages.

Currently, this process stops after reaching a threshold based on the
message limit (`maxmsg`) configured on the current folder. However, if
`maxmsg` is lowered when a voicemail folder contains more than
`maxmsg + 10` messages, resequencing will not run completely leaving
the mailbox in an inconsistent state.

We now resequence up to the maximum number of messages permitted by
`app_voicemail` (currently hard-coded at 9999 messages).

Fixes #86