Stdio.File: Support buffering in write().

Stdio.File()->write() now buffers in the output buffer if there is one,
and then to the actual fd.
release number bumped to 8.0.6 by export.pike
release number bumped to 8.0.7 by export.pike
Some changes to CHANGES.

Mainly rewording things, added some notes about Stdio.Buffer
Mysql.list_fields(): support more field flags
Nettle: moved rsa_unpad() to nettle.cmod

rsa_unpad() does not actually depend on hogweed and is used without it.
This fixes the Crypto module when compiled with old nettle versions
(without hogweed).
CHANGES: one typo
Fix compile errors in emulated accept4() added in 236e8efea9.
Fix a bunch of Clang warnings.
Fixed yet another Clang warning.
Fix "make bin_export" on OS X to not include phantom resource fork file.