strings/memory: constify and func_attr.

Fix MB_COPY_cHAR() to accept const pointers.
strings: Enable -Wconvert.
Wconversion: fix #1578
Merge pull request #1497 from splinterofchaos/const-attr

constify and func-attribute memory.c and strings.c
Fix SegFault when getting full path for files
Merge pull request #1582 from LBEaston/FixSegFault

Fix SegFault when entering Command(q:) or Search(q/) History
Remove BeOS/Haiku references
Platform cleanup: remove unsupported MS platform refs

Remove references to:
- msdos
- dos16
- dos32
- win16
- win32s
- DJGPP
- Windows 3.1
- Windows 3.11