Fixes #6. Change-Id: Id9950f59c7970575866a7c22a69bfbf3a271f2bb

Before, executing Get in a file rewound the window offset and selection to the start of the file. After this CL, Get preserves the window offset and selection, where preserve is defined as "the same line number and rune offset within the line". So if the window started at line 10 before and the selection was line 13 chars 5-7, then that will still be true after Get, provided the new content is large enough. This should help the common situation of plumbing a compiler error, realizing the window is out of date, clicking Get, and then losing the positioning from the plumb operation.

This PR adds additional files to update /usr/local/plan9 references for packaging.

Thanks to Lorenzo Beretta for noticing.

Fixes #114.

Since macOS 10.13, opening the /dev/ptyXX files always return ENOENT. Consequently, we changed getpts to use openpty to open a pseudoterminal, like on Linux and OpenBSD. Fixes #90. Fixes #110.

decode.c:146:8: warning: variable ‘argv’ set but not used fs.c:953:47: warning: variable ‘reset’ set but not used imap.c:348:6: warning: variable ‘prefix’ set but not used Updates #114.

Bad remote file systems can change mtime unexpectedly, and then there is the problem that git rebase and similar operations like to change the files and then change them back, modifying the mtimes but not the content. Avoid spurious Put errors on both of those by checking file content. (False positive "modified since last read" make the real ones difficult to notice.)

After making the build on macOS silent on commit 310ae03, the build was broken on macOS lesser than 10.12 (Sierra). This commit conditionally checks the version the of the SDK before using the defined values for silent build. Fixes #66.

Ported from Plan 9 2013-05-21. https://github.com/0intro/plan9/commit/b377a116d132865c011a3fb3ea76528ffd32963f Closes #16.

On FreeBSD, the Chromium executable is called chrome. Fixes #108.

Fixes #78.