[R/write] add na.strings argument to write functions

[JanMarvin]

Add option to write custom na.strings. It is na.strings even though it allows just a single value, to match the identical argument when reading. This allows three options if na.strings

• is missing: write #N/A exception to output
• is NULL: write nothing
• is character or numeric: write inlineStr of as.character(na.strings)

This applies to NA only and is intended to be used when replacing NA in numeric data with character strings or empty cells.