sh(1): Allow non-printing characters in prompt strings

Introduce new prompt format characters:

• '\[' starts the sequence of non-printing chatacters
• '\]' ends the sequence of non-printing characters

Within these sequences, the following characters are now supported:

• '\a' emits ASCII BEL (0x07, 007) character
• '\e' emits ASCII ESC (0x1b, 033) character
• '\r' emits ASCII CR (0x0d, 015) character
• '\n' emits ASCII CRLF sequence

These can be used to embed ANSI sequences into prompt strings.

Example in .shrc:

PS1="\[\e[7m\]\u@\h\[\e[0m\]:\w \\$ "

This tries to maintain some degree of compatibility with GNU bash,
that uses GNU readline library (which behaves slightly different from
BSD editline): It has two "non-printing boundary" characters:

• RL_PROMPT_START_IGNORE (\001)
• RL_PROMPT_END_IGNORE (\002)

while BSD editline only has one (when using EL_PROMPT_ESC setting), so
for this purpose, ASCII \001 was chosen and both \[ and \] emits
this character.

And while here, enlarge PROMPTLEN from 128 to 192 characters.

Reviewed by: jilles
Approved by: jilles
Differential Revision: https://reviews.freebsd.org/D37701