fix(darwin): quote security -i arguments without a shell escaper

[SashaMIT]

Set builds its security -i line with shellescape.Quote. That wrapper is for a POSIX shell. security -i is not one: a backslash takes the next byte literally, including inside single quotes.

A service a\b is stored as ab. Set returns success, and the matching Get returns not found. A single quote produces a line the parser rejects with exit 2.

Each value is now wrapped in double quotes, with backslash and double quote escaped. On a throwaway keychain, 'a\b' is stored as ab and "a\\b" is stored as a\b. A single quote, a double quote, a space, and an empty value round-trip. The POSIX form 'a'"'"'b' still exits 2.

go test -count=1 . passed. Pointing the quoter back at shellescape.Quote fails TestSecurityInteractiveQuote.

The length check in #156 still calls shellescape.Quote on this same line. That call needs this quoter when the two changes meet.

Fixes #154