ssmtp truncates headers longer than BUF_SZ

[Syed-Shahrukh-OSSRevival]

Reported by: herbert@gondor.apana.org.au

The function smtp_write fails to handle headers longer than BUF_SZ.
For example, the following email will be truncated, resulting in an
invalid email header:

https://lore.kernel.org/all/de35edd9-b85d-0ed7-98b6-7a41134c3ece@foss.st.com/

There is no reason why this limit should exist. This patch fixes
it so that the length is practically unlimited (it is still limited
by the return value of vsnprintf which is INT_MAX).

buffer.patch