warning C4244: '=': conversion from 'double' to 'uint64_t', possible loss of data

This is a millisecond timer that tries to avoid floating-point
operations.

Now it should keep working after 24 days.

than the int supports, it's set to 0x80000000 to indicate overflow.

msclock() is *always* overflowing, and clock_t is only 32-bits on
some platforms (specifically FreeBSD).  To "avoid" problems, just
keep subtracting UIN32_MAX from the value until it's less than INT_MAX
then cast.

This function is, of course, terrible and shouldn't actually be used,
but it should at least sorta kinda workish.

At least one of MAP_ANON, MAP_GUARD, MAP_PRIVATE, MAP_SHARED, or MAP_STACK
must be specified.

For some reason this deprecation warning is being treated as an error (and only with newer Windows platform SDKs). Just disable this warning for the GetVersionEx line.
See https://stackoverflow.com/questions/22303824/warning-c4996-getversionexw-was-declared-deprecated for details.

This change needed to build dosxtrn.exe (again).

Now correctly rejects UTF-8 encoded Unicode surrogates and does not support
5 and 6 byte UTF-8 encodings.

For reference:
https://github.com/openssl/openssl/commit/ba64e5a92a6f009e311ad1c3565817820a1632a4

Per https://man7.org/linux/man-pages/man3/pthread_yield.3.html
       This call is nonstandard, but present on several other systems.
       Use the standardized sched_yield(2) instead.

Should fix #299.

write()
chsize()

Fix FreeBSD build error:
dirwrap.c:760:13: error: use of undeclared identifier 'GLOB_PERIOD'
Not sure how glob() on FreeBSD treats "dot files" or how to control that.

glob() by default, doesn't include 'dot files', so include the GLOB_PERIOD
flag so that when deleting all files in a directory (e.g. node1/temp/*), any
files starting with a dot will be deleted too.

Now IS_WHITESPACE() will return true for that char and this allows things like truncsp() to truncate trailing white-space (e.g. from a FILE_ID.DIZ), even if it contains a terminating 0xFF character for some unknown reason.

SBBSecho uses mkstemp() now (instead of tempnam()) - so we need this POSIX function wrapper for Windows builds to succeed.

One in an error path (CID 331353), the other not.

This also exposed problems with the find_section() utility function: when the "found" section was actually empty and the next immediate line in the ini file was the beginning of a new section, the "keys" indexed would actually be the next section. Now fixed.

Pretend we are RipTel v3.
Implement $PCB$
Implement click actions
Fix M areas.

Return int value for "string not found" rather than crash/segfault. Seems the better option.