Constify where possible.

Approved by:	cperciva (mentor)
Due to calendar(1)'s abuse of the C preprocessor "unix" gets expanded to
"1".  'Fix' this.

PR:		bin/175790
Submitted by:	ak
Approved by:	cperciva (mentor)
Fix typo ('1' != 'l')

PR:		bin/175975
Submitted by:	William Ahern <william@25thandclement.com>
Approved by:	cperciva (mentor)
devd: Use volatile sig_atomic_t for the flag set by a signal handler.

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
devd: Remove call to _exit() from signal handler, which also sets a stop flag.

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
devd: Simplify while (1) { if (x) break; } to while (!x) {}.

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
devd: Use the standard constructor of std::string instead of string("").

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
devd: Avoid unnecessary temporary objects (and simplify the code) when handling std::string.

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
devd: Remove empty virtual destructor from class, which has noch subclasses.

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
devd: Remove unnecessary empty default constructors.

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
devd: Use string::empty() instea of string::length() == 0.

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
devd: Use simpler dst += *x instead of str.append(x, 1).

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
devd: Correct typo in comment.

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Approved by:	cperciva (mentor)
Remove check for NULL prior to free(9) and m_freem(9).

Approved by:	cperciva (mentor)