AppendWinMsgRend is unused
MakeWinMsgEv now using AddWinMsgRend

Cutting down on duplicate code to make unicode changes less bug-prone
Replaced magic padding value 127 with constant CHRPAD
execvpe defined in unistd.h
Began refactoring winmsg code into its own file

This is really to prepare for refactoring MakeWinMsgEv, which is rather large
and unnecessarily difficult to follow; this will make me much more confident
that I actually grok the code before proceeding with the unicode changes and
will further help others getting into the project.
Refactored rendition start handling into WinMsgRend

Note that the inline keyword need not be included in these function
definitions, as -finline-functions-called-once is enabled automatically at
-O1 and up.
Added undocumented %p escape to texinfo doc
Refactored %p into WinMsgPid
Refactored %P into WinMsgCopyMode
Began adding winmsg escape handler definition and invocation abstractions as macros
Began refactoring variables in MakeWinMsgEv into a WinMsgEsc struct to describe escape sequences
Refactored %w and %W into WinNames escape handler
Added %x and %X escapes to texinfo doc (was missing)
Refactored %x and %X escapes into WinArgv handler
Corrected display bug with %x and %X escapes when !win

The escape character was being output.
WINMSG_* => WINESC_* for WinMsgEscapeChar
Refactored %f escape into Wflags handler
Refactored %S escape sequence into SessName handler
Refactored %H escape sequence into HostName handler
Refactored %F escape sequence into Focus handler