Fix improper signed int conversion of IMAP uid and msn values.

Fix improper signed int conversion of IMAP uid and msn values.

Several places in the imap code, when parsing "number" and "nz-number"
values from the IMAP data, use atoi() and strtol().  This is
incorrect, and can result in failures when a uid value happens to be
larger than 2^31.

Create a helper function, mutt_atoui() and use that instead.  One
place was using strtol() and relying on the endptr parameter, and so
was changed to use strtoul() instead.

Thanks to Paul Saunders for the bug report and original patch, which
this commit is based on.