imap-send.c: use less verbose strbuf_fread() idiom
When looking for things that hardcoded a non-zero "hint" parameter to
strbuf_fread() I discovered that since
f2561fda364 (Add git-imap-send,
derived from isync 1.0.1., 2006-03-10) we've been passing a hardcoded
4096 in imap-send.c to read stdin.
Since we're not doing anything unusual here let's use a less verbose
pattern used in a lot of other places (the hint of "0" will default to
8192). We don't need to take a FILE * here either, so we can use "0"
instead of "stdin". While we're at it improve the error message if we
can't read the input to use error_errno().
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>