cosash: Ensure we always inject trailing newline into commands

[cgwalters]

The HasPrivileges() API would hang forever because it didn't have a trailing newline in the command emitted. This took me some time to figure out. Let's fix this by auto-injecting a trailing newline automatically to entirely avoid the problem.

[cgwalters]

A side note, I discovered while using strace on this that bash does 1 byte length reads on stdin...the inefficiency in this in a post-spectre world is high. Of course what we're doing here isn't high performance...

$ strace -f -e read bash -c "echo 'echo hello' | bash"
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\20\227\2\0\0\0\0\0"..., 832) = 832
read(3, "# Locale name alias data base.\n#"..., 4096) = 2998
read(3, "", 4096)                       = 0
strace: Process 3410022 attached
strace: Process 3410023 attached
[pid 3410022] +++ exited with 0 +++
[pid 3410023] read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 832) = 832
[pid 3410023] read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\20\227\2\0\0\0\0\0"..., 832) = 832
[pid 3410023] read(3, "# Locale name alias data base.\n#"..., 4096) = 2998
[pid 3410023] read(3, "", 4096)         = 0
[pid 3410023] read(0, "e", 1)           = 1
[pid 3410023] read(0, "c", 1)           = 1
[pid 3410023] read(0, "h", 1)           = 1
[pid 3410023] read(0, "o", 1)           = 1
[pid 3410023] read(0, " ", 1)           = 1
[pid 3410023] read(0, "h", 1)           = 1
[pid 3410023] read(0, "e", 1)           = 1
[pid 3410023] read(0, "l", 1)           = 1
[pid 3410023] read(0, "l", 1)           = 1
[pid 3410023] read(0, "o", 1)           = 1
[pid 3410023] read(0, "\n", 1)          = 1
hello
[pid 3410023] read(0, "", 1)            = 0
[pid 3410023] +++ exited with 0 +++