Return coding-system corresponding to CHARSET.
CHARSET is a symbol naming a MIME charset.
If optional argument LBT (`unix', `dos' or `mac') is specified, it is
used as the line break code type of the coding system.
If ALLOW-OVERRIDE is given, use `mm-charset-override-alist' to
map undesired charset names to their replacement. This should
only be used for decoding, not for encoding.
A non-nil value of SILENT means don't issue a warning even if CHARSET
is not available.