Remove lock recursion for the pipe pair mutex, and disable the
recursion on mutex initialization.

The only places where the recursive acquire is performed are read and
write filters, since knlist, which uses the pipe pair mutex as lock,
is locked when filter is called.

The recursion was added in r93296, and consistent locking for
kn_fop->f_event() introduced in r133741.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 month
The current limit of 100k for the linker hints file is getting a bit
crowded as we now are at about 70k. Bump the limit to 1MB instead
which is still quite a reasonable limit and allows for future growth
of this file and possible future expansion to additional data.

MFC After: 2 weeks
Update _ENTRY to use _EENTRY to reduce the common code.
Add support for dtrace:fbt on modules for PowerPC

Summary:
Revert the initial FBT-with-KDB changes for trap_subr*.S, and instead use the
db_trap filter function to handle dtrace trap filtering.  With this, the MMU is
enabled by the support code, simplifying the codepath altogether.

Test Plan: Tested on my G4 PowerBook

Reviewers: #powerpc, nwhitehorn

Reviewed By: nwhitehorn

Differential Revision: https://reviews.freebsd.org/D1207

MFC after:	3 weeks
etc/mail actually applies to MK_SENDMAIL, not MK_MAIL
Revert r275270

MK_MAIL applies to MK_SENDMAIL and a number of other knobs. r275270 incorrectly
applied it to only MK_SENDMAIL