Fix the following -Werror warning from clang 3.5.0, while building the
ath kernel module:

sys/dev/ath/ath_hal/ar5212/ar5212_reset.c:2642:7: error: taking the absolute value of unsigned type 'unsigned int' has no effect [-Werror,-Wabsolute-value]
                if (abs(lp[0] * EEP_SCALE - target) < EEP_DELTA) {
                    ^
sys/dev/ath/ah_osdep.h:74:18: note: expanded from macro 'abs'
#define abs(_a)         __builtin_abs(_a)
                        ^
sys/dev/ath/ath_hal/ar5212/ar5212_reset.c:2642:7: note: remove the call to '__builtin_abs' since unsigned values cannot be negative
sys/dev/ath/ah_osdep.h:74:18: note: expanded from macro 'abs'
#define abs(_a)         __builtin_abs(_a)
                        ^
1 error generated.

This warning occurs because both lp[0] and target are unsigned, so the
subtraction expression is also unsigned, and calling abs() is a no-op.

However, the intention was to look at the absolute difference between
the two unsigned quantities.  Introduce a small static function to
clarify what we're doing, and call that instead.

Reviewed by:	adrian
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D1212
\n at end of panicstr is redundant.

Submitted by:	alc
Consider the negation operator (!) to be a word even if it is not followed
by whitespace.  This allows "optional !foo" which is what most programmers
are naturally going to tend to do as opposed to "optional ! foo".
Misc mdoc fixes:

- Remove superfluous paragraph macros.
- Remove/fix empty or incorrect macros.
- Sort sections into conventional order.
- Terminate quoted strings properly.
- Remove EOL whitespace.
Squelch a (bogus) used before init warning when building with gcc.
Fix an error (case on a .Nm) preventing mandoc from rendering the manpage
Fix mandoc warnings: extrat space at end of lines
Correct the functions declaration.
Regenerate the manpage in mdoc(7) format using pod2mdoc

Now mandoc can render the manpage properly
Install mdoc(7) and man(7) from mdoc instead of the one from groff