wmbiff: handle EAGAIN or GNUTLS_E_AGAIN
From Debian bug #917467, reported by Nye Liu <nyet@nyet.org> [1]:
If gnutls_read() or read() report EAGAIN, tlscomm_expect() fails:
wmbiff/nyet comm: wrote a000 CAPABILITY
wmbiff/nyet comm: imap.***.***:993: expecting: * CAPABILITY
wmbiff/nyet comm: imap.***.***:993: gnutls error reading: Resource temporarily unavailable, try again.
wmbiff/nyet imap4: unable to query capability stringwmbiff/nyet comm: wrote a002 LOGOUT
wmbiff/nyet comm: imap.***.***:993: closing.
[1] https://bugs.debian.org/917467