ed: factor r command out of edEdit()

[mknos]

• Edit (e) command has a forceful alternative (E), but read (r) command doesn't (since r doesn't destroy the buffer)
• edEdit() continues to handle e and E ($QuestionsMode param determines the mode)
• r and e commands both use the saved filename if no argument is provided
• Command arguments starting with '!' now call function init_pipe(), which rejects NUL and provides an argument list to pipe form of open()
• Also add helper functions open_file_ro() and readin_lines() for common code
• r command never updates $RememberedFilename; e command updates it if a file argument was provided
• test1: "e !this" --> bad command, show error, buffer unchanged
• test2: "r !this" --> same as test1
• test3: "e !ls -l" --> buffer replaced with ls output if buffer is clean
• test4: "E !echo E" --> buffer replaced with echo output unconditionally
• test5: "1,2e a.s" --> invalid command, e doesn't take any addresses
• test6: "0r a.s" --> file a.s is read into the beginning of the buffer

[coveralls]

Pull Request Test Coverage Report for Build 12844597261

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 73.389%

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Totals
Change from base Build 12843752400:	0.0%
Covered Lines:	353
Relevant Lines:	481

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 12844597257

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 74.12%

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Totals
Change from base Build 12843752400:	0.7%
Covered Lines:	358
Relevant Lines:	483

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💛 - Coveralls