Operation on read-only filesystem

[Patrick Higgins]

With the new lock-never option, I can almost use GPG on a read-only
filesystem.  However, in tdbio.c the key databases are opened with the
O_RDWR flag, which fails on a read-only filesystem.  After patching my
source, things seem to run without trouble.  Perhaps a new option
(--read-only?) which would allow such operation without patching would be
useful?

Also, in order to do this, I have to use the no-random-seed-file option,
which is documented as hurtful to performance.  How bad is this hit?  I'm
using pretty slow hardware as it is, and I'd like to avoid serious
performance hindering options.

--Patrick