Restore the traditional behaviour of the man(7) single-font

Restore the traditional behaviour of the man(7) single-font
macros .B, .I, .SM, and .SB that the next-line scope extends
to the end of the next logical input line and is not extended
if that line ends with a \c (no-space) escape sequence.

While improving a loosely related feature in the man(7) .TP
macro, a regression entered the groff codebase in groff
commit 3549fd9f (28-Apr-2017) caused by the usual sloppiness
of Bjarni Ingi Gislason.  Since that time, groff wrongly had \c
extend next-line scope to a second line for these macros.
In man.c rev. 1.127 (25-Aug-2018) i synched mandoc behaviour
with groff in this respect, unfortunately failing to notice
the recent regression in groff.  The groff regression was
finally fixed by gbranden@ in commit 09c028f3 (07-Jun-2022).

With the present commit, mandoc is back in sync with both GNU and
Heirloom roff regarding the interaction of single-font macros with \c.